The niobium aluminide Al.sub.3 Nb has no detectable homogeneity range, is congruently melting, and has the DO.sub.22 crystal structure similar to Al.sub.3 Ti. Available information on Al.sub.3 Nb concerns its use as an oxidation-resistant coating for niobium-based alloys. Coatings of Al.sub.3 Nb can be formed by dipping the niobium alloy into a bath of molten aluminum, which may contain small additions of elements such as chromium and silicon to improve coating performance. The Al.sub.3 Nb outer layer provides a thin, protective layer of Al.sub.2 O.sub.3 on the coated substrate.
Because the Al.sub.3 Nb phase has a high melting point (above 1600.degree. C.), low density (comparable to that of titanium), and general oxidation resistance, it has potential as a high-temperature structural material. However, its use is severely limited by a lack of ductility because of the DO.sub.22 crystal structure and the great difficulty in forming the material. Further, the related refractory aluminides such as those of tungsten and tantalum, and the more complex refractory ternary aluminides can be produced to have more favorable crystal structures with improved ductility. These compounds, such as NbTiAl.sub.3, Nb.sub.2 Zr.sub.3 Al, and NbVAl.sub.2, may also have higher melting temperatures, wider ranges of homogeneity and improved oxidation resistance relative to the binary aluminide phase. These compounds are extremely difficult to produce, however, using conventional casting and solidification processes because of segregation of the various elements. Applicant has found, however, that refractory aluminides can be formed by high temperature direct reaction of aluminum and a refractory metal. This reaction is accomplished at elevated temperatures above the melting point of aluminum. The reaction occurs with the aluminum in the liquid phase and results in the direct formation of the refractory aluminide. The reaction, once initiated is exothermic and proceeds to completion if permitted to sustain. The formed refractory aluminide may be recovered and may be densified by pressure treatment under elevated temperature. Densities of substantially theoretical levels can be achieved. Stock and near net shape parts may be formed by conducting the reaction is a shaped die.
It is thus an object of applicant's invention to provide a process of producing refractory aluminides.
It is a further object of applicant's invention to produce refractory aluminides by direct reaction of aluminum and refractory metals.
It is a further object of applicant's invention to produce refractory aluminides by reacting liquid aluminum with refractory metals.
It is a further object of applicant's invention to produce a dense, homogeneous, single phase Al.sub.3 Nb alloy.